The subject invention generally relates to ink jet printing, and more particularly to a thin film ink jet printheads for ink jet cartridges and methods for manufacturing such printheads.
The art of ink jet printing is relatively well developed. Commercial products such as computer printers, graphics plotters, and facsimile machines have been implemented with ink jet technology for producing printed media. The contributions of Hewlett-Packard Company to ink jet technology are described, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985); Vol. 39, No. 5 (October 1988); Vol. 43, No. 4 (August 1992); Vol. 43, No. 6 (December 1992); and Vol. 45, No. 1 (February 1994); all incorporated herein by reference.
Generally, an ink jet image is formed pursuant to precise placement on a print medium of ink drops emitted by an ink drop generating device known as an ink jet printhead. Typically, an ink jet printhead is supported on a movable print carriage that traverses over the surface of the print medium and is controlled to eject drops of ink at appropriate times pursuant to command of a microcomputer or other controller, wherein the timing of the application of the ink drops is intended to correspond to a pattern of pixels of the image being printed.
A typical Hewlett-Packard ink jet printhead includes an array of precisely formed nozzles in an orifice plate that is attached to an ink barrier layer which in turn is attached to a thin film substructure that implements ink firing heater resistors and apparatus for enabling the resistors. The ink barrier layer defines ink channels including ink chambers disposed over associated ink firing resistors, and the nozzles in the orifice plate are aligned with associated ink chambers. Ink drop generator regions are formed by the ink chambers and portions of the thin film substructure and the orifice plate that are adjacent the ink chambers.
The thin film substructure is typically comprised of a substrate such as silicon on which are formed various thin film layers that form thin film ink firing resistors, apparatus for enabling the resistors, and also interconnections to bonding pads that are provided for external electrical connections to the printhead. The thin film substructure more particularly includes a top thin film layer of tantalum disposed over the resistors as a thermomechanical passivation layer that protects against cavitation damage.
The ink barrier layer is typically a polymer material that is laminated as a dry film to the thin film substructure, and is designed to be photodefinable and both UV and thermally curable.
An example of the physical arrangement of the orifice plate, ink barrier layer, and thin film substructure is illustrated at page 44 of the Hewlett-Packard Journal of February 1994, cited above. Further examples of ink jet printheads are set forth in commonly assigned U.S. Pat. No. 4,719,477 and U.S. Pat. No. 5,317,346, both of which are incorporated herein by reference.
Color ink jet printers commonly employ a plurality of printheads mounted in the print carriage to produce a full spectrum of colors. For example, in a printer with four printheads, each printhead can provide a different color output, with the commonly used base colors being cyan, magenta, yellow and black. In a printer with two printheads, one printhead provides a black output, while the other provides cyan, magenta and yellow outputs from respective nozzle sub-arrays.
The base colors are produced on the media by depositing a drop of the required color onto a pixel location, while secondary or shaded colors are formed by depositing multiple drops of different base colors onto the same or an adjacent pixel location, with the overprinting of two or more base colors producing the secondary colors according to well established optical principles.
In order to achieve photographic-like quality color printing in four ink printing systems, ink drop volume needs to be reduced significantly, for example to about 3 picoliters, wherein non-photographic quality four ink systems commonly operate with a drop volume of about 30 picoliters. While the above-described ink jet printhead architecture has been adapted for reduced drop volumes by shrinking the resistor, chamber and nozzle dimensions, there is in the reduced size printhead architecture a significant increase in "kogation" which is the accumulation of a ink components that are tenaciously adhered to the tantalum passivation layer in the ink chambers. Such kogation layers reduce the heat transfer to the ink during a firing event, which in turn leads to smaller, slower, and often misdirected drops. Eventually, an affected nozzle will fail.
The problem of kogation at lower ink drop volumes has been addressed by alterations to ink chemistry such as the addition of anionic phosphates. However, the phosphate additions do not prevent kogation with many dyes, and force trade-offs in other ink attributes such as dry time, waterfastness and light fastness.
The problem of kogation has also been addressed by increasing drop volume relative to optimal drop volumes. This however causes unacceptable print quality degradation.
Accordingly, there is a need for a non-kogating low drop volume ink jet printhead.